Wind turbines are well known devices used to extract energy from the wind. Most commonly these are horizontal axis machines with two or more blades. A key design challenge for all such machines is the need to regulate the power extracted from the wind to avoid excessive rotor speed or overload of the generator in high winds. One method of achieving this is through altering the pitch angle of the blades. This can be achieved either through active regulation (for example using electrical or hydraulic actuators) or passive means. Passive regulation involves the use of the forces that are naturally present within the wind turbine to pitch the blades against a spring or deform the blades or their attachment points.
The forces that are naturally present include the centrifugal loads acting on the wind turbine blades, the torque in the main shaft and the thrust load acting axially along the main shaft. These forces may be used in many different physical configurations to pitch the blades. Pitching of the blades may either be towards feather (to reduce the aerodynamic angle of attack and therefore reduce the aerodynamic lift force that is providing the power to the wind turbine) or towards stall. Pitching towards stall increases the angle of attack and can therefore increase power initially, but eventually the pitch angle will be reached where the blades will ‘stall’ and the aerodynamic lift will be lost and the aerodynamic drag will increase. Thus the wind turbine will lose its ability to produce power.